Starch Allowance and Muscle Enzyme Activity in Healthy Standardbred Trotters Trained by Professional Trainers

It is generally accepted that plasma muscle enzyme activity of creatine kinase (CK) and aspartate aminotransferase (AST) may increase in racehorses after exercise and racing, indicating muscle fibre damage and/or increased leakage from muscle fibres. However, other studies suggest that starch intake might influence plasma muscle enzyme activity reported postexercise. This study aimed to evaluate the effect of different starch allowances on plasma muscle enzyme activity in Standardbred trotters in professional training. Seventy-six horses from five professional trainers were sampled pre- and postexercise. The trainers had different feeding strategies and fed various amounts of starch to their horses. Postexercise plasma AST activity was higher (p < 0.007) for the high (H) and medium (M) starch allowances (451-967 and 988-1429 g/day, respectively) than for horses with low starch allowances (L) (0-268 g/day) (H:8.1 ukat/L (SE 0.5); M: 8.4 ukat/L (SE 0.5); L: 5.8 ukat/L (SE 0.7) (p < 0.007)). Postexercise plasma CK activity was higher (p < 0.01) for the medium starch allowance group than for the high and low starch allowance groups (H:4.6 ukat/L (SE 0.3); M: 5.9 ukat/L (SE 0.4); L: 3.9 ukat/L (SE 0.4)). In conclusion, this study showed higher plasma muscle enzyme activity of AST and some elevations in CK activity in horses fed high-starch allowances compared to horses fed low allowances or no starch. In addition, muscle enzyme activity increased in response to the duration of high-intensity exercise. Management systems aiming for low levels of plasma muscle enzyme activity could accordingly offer diets with low starch (< 450 g/day) contents and perhaps training regimes with shorter durations of high-intensity exercise. However, the mechanisms behind and the elevations' biological importance, need further investigation.